L5 - Chloroplasts: Structure and Function

Question 1

What are the origins of plastids?
Give three points of evidence

Answer 1

• Result of symbiosis after a eukaryote engulfed a cyanobacteria-like photosynthetic prokaryote.

Evidence:
1) Double membrane - either one from the the eukaryote and one from the prokaryote or both from the the prokaryote (inner and outer membrane as seen in modern cyanobacteria) e.g. Glaucocystophytes contain peptidoglycan cell wall.

2) Similar DNA to Cyanobacteria - similar sequence, double stranded and circular.

3) Similar promoters, ribosomes and division to bacteria

Question 2

Outline the evolutionary theory for the presence of plastids in multiple plant groups. Name these groups and give evidence.

Answer 2

• One original symbiotic event followed by rapid divergence (evidence previously given) giving red, green algae and glaucocystophytes.
• Secondary and tertiary symbioses followed.
  Evidence:
  1) Nucleomorph in some algae
  2) Additional membrane

Question 3

Give 4 similarities and 4 differences between chloroplasts and their cyanobacterial ancestors.

Answer 3

Similarities:
1) Plastocyanin in thylakoid lumen
2) LHCs and RCs in thylakoid membrane
3) RuBisCO, Starch, Oil, DNA, ribosomes in the stroma.
4) Similar DNA: Circular, double stranded, conserved sequence.

Differences:
1) Outer membrane envelope proteins seen - import proteins from cytoplasm
2) Inner membrane transporters seen - export photosynthetic products to cytoplasm. E.g. TPT
3) Only 5-10% genes of free cyanobacteria. Most genes migrated to the nucleus (E.g. 18% nucleus genes in Arabidopsis derived from cyanobacteria, mostly not for chloroplast)

Question 4

Give the average number of chloroplasts per mesophyll cell.

Give one line of evidence supporting the claim of similar division between chloroplasts and bacteria

Answer 4

• 50 to 100 chloroplasts per mesophyll cell
• Filamentous temperature sensitive (fts) mutants seen in E. coli that couldn’t divide.
• Gene FtsZ found largely responsible for fts.
• FtsZ homologue found in Arabidopsis nucleus.
• Antisense FtsZ seen to inhibit division in Arabidopsis.

Question 5

Outline another similar gene seen in bacteria and chloroplast division.

Answer 5

• Other bacteria mutants didn’t divide symmetrically, forming minicells.
• MinD gene identified as controlling symmetrical division.
• MinD w/ florescent reporter in E.Coli oscillated from pole to pole, inhibiting division in poles.
• Homologue of MinD in Arabidopsis. Altering MinD levels changes chloroplast shape.

Question 6

Is having correctly sized chloroplasts needed? Why?

Answer 6

• Yes, FtsZ mutants with altered chloroplast division and size grow less well than WT.
• Not able to move plastids around cytoskeleton to cope with changes in light supply as wrong size.
• This causes photodamage and inefficient positioning.

Question 7

Outline the mechanism by which chloroplasts are moved around the cytoskeleton and the evidence.

Answer 7

• Chloroplasts moved in response to blue light.
• Blue light receptors communicate to cytoskeleton which tethers chloroplasts.
• Moved vertical in high light - avoid damage.
• Highly conserved mechanism between species.
• cav1 mutant lacks blue light receptor and doesn’t move chloroplasts in high light.

Question 8

Give the 5 main plastid types and where they’re found in the plant.

Answer 8

1) Proplastid - Meristematic cells, specialises into other plastid depending on conditions.
2) Chloroplast - Leaves
3) Leucoplasts - Root cells
4) Chromoplasts - Petals and fruit (store pigment)
5) Amyloplasts - Tubers (store starch)

Question 9

Are plastids unique to plants? Give examples if not.

Answer 9

• No
• Found in Plasmodium falciparum, the malaria parasite.
• Found in some marine mollusc stomachs.

Question 10

Explain the character of plastids

Answer 10

• Extremely dynamic
• Transient stroma filled tubules called stromules formed between plastids